Many viral infections induce liver pathology, and the association of infection, by hepatitis B virus and hepatitis C virus, with the development of cirrhosis and hepatoma in humans, highlights the need to understand mechanisms regulating pathogenesis of viral hepatitis. Available evidence suggests that virus-induced hepatitis is related to an ineffective innate immune response;however, the exact cellular and molecular mechanisms leading to establishment of an infection of the liver and the development of pathology remain elusive. We propose to use the murine coronavirus, mouse hepatitis virus (MHV) that induces hepatitis in a strain specific manner, to evaluate both viral genetic determinants and host intrahepatic responses underlying the development of hepatitis. MHV-A59 is an hepatotropic strain and MHV-JHM is a highly neurotropic strain that fails to infect the liver even when inoculated at very high doses. Perhaps surprisingly, both A59 and JHM replicate successfully in primary hepatocytes in culture. Using these strains as well as chimeric A59/JHM recombinant viruses, we have previously established that the genes in the 3[unreadable] one third of the MHV genome determine the ability to replicate in the liver and the corresponding severity of liver damage. More recently we have found that the accessory ns2 protein of A59 is a type I interferon antagonist;mutation of this protein confers attenuation of replication in bone marrow derived macrophages and abrogates the ability of A59 to induce hepatitis. Taking these data together, we have formulated the overall hypothesis that MHV hepatotropism is a result of an early host response that differs during A59 versus JHM infection and that the development of hepatitis requires the function of multiple viral genes, including ns2, and the ability to antagonize type I IFN signaling in macrophages. We will test this hypothesis with the following aims: 1) Using reverse genetics, we will select A59/JHM chimeric recombinant viruses to map the genes necessary and sufficient for the induction of hepatitis;we will test the hypothesis that liver sinusoidal endothelial cells and/or Kupffer cells restrict entry and/or replication in the liver parenchyma and characterize the cytokine/chemokine environment in the infected liver to identify factors contributing to the pathogenesis of hepatitis;2) We will investigate the mechanism by which ns2 enhances the ability of A59 to antagonize the effects of type I interferon signaling, using primary cultures of liver cells and mice depleted of macrophages or ablated for interferon signaling specifically in macrophages and 3) We will assess the mechanisms by which MHV accelerates downregulation of type I IFN receptor (IFNAR) on the cell surface and use mice in which IFNAR has been stabilized on the cell surface to determine the role of IFNAR downregulation in hepatovirulence. Elucidation of the mechanisms that underlie MHV induced hepatitis will contribute to the understanding of the virus-host interactions that lead to the development of hepatitis and in the long term lead to the design of vaccines and anti-viral therapies, in particular refinement of the use of IFN-&#945;as therapy for human hepatitis.